The present invention relates to digital communication techniques, and more particularly to the reception of data blocks into which are inserted error detection codes.
It is very usual to make provision for such codes in transmitted data block formats. They are generally codes appended to the data of the block by applying a systematic cyclic coding. One speaks of parity checking or CRC (“cyclic redundancy checksum”) codes.
The invention is more particularly aimed at systems where provision is made for at least two codes of this kind within a data block, to check the integrity of distinct parts of the block: a header and one or more data fields.
Often, the information contained in the header of a block is more important than that contained in the data fields. The header may thus specify information useful for the reception of the remainder of the data of the block or for the proper operation of the receiver (identification of the recipient of the block, format of the data, modulation, error correcting coding applied, information useful for the link layer protocols, etc). To further protect this information, a coding scheme that is more robust to errors is applied to the header and the former is furnished with a CRC distinct from that used for the data field.
Such a distinction is for example envisaged for the transmission of data in the cellular radio communication system of EGPRS type (“Enhanced General Packet Radio Service”) standardized by the ETSI (“European Telecommunications Standards Institute”). This system makes it possible to transmit, between a base station and a mobile station, modulated-signal blocks that may form the subject of several modulations and of several transmission error correcting codings. The modulation and the coding are selected block by block by choosing a Modulation and Coding Scheme MCS from among nine schemes MCS-1 to MCS-9 specified in the ETSI EN 300 909 European standard, Digital cellular telecommunications system (Phase 2+); Channel coding (GSM 05.03, version 8.5.1, Release 1999) published by the ETSI in November 2000. Each of these schemes offers a level of data protection that is more or less robust and correspondingly a bigger or smaller information bit rate.
The header of an EGPRS radio block comprises an 8-bit CRC, while its data field comprises a 12-bit CRC.
The choice of the modulation and coding scheme applied to a block results from a link adaptation algorithm whose input variables comprise, for each mobile station, parameters representing the quality of reception observed by the receiver (the mobile station in the downlink direction or the base station in the uplink direction). Mediocre quality (high bit error rate) attests to difficult propagating conditions, and requires an increase in the protection against errors for the mobile concerned and hence a reduction in the useful bit rate.
The presence of a block originating from a given mobile station or destined for the latter is decided by the receiver when the CRC of the header is correct and when the content of the header designates this mobile station. This decision makes it possible to update the relevant quality estimators.
The size of the CRC of the header (8 bits) gives rise to a false detection probability of 1/256, i.e. around once every 5 seconds. When a false detection such as this occurs, the link adaptation relating to the mobile station designated in the header is disturbed. In particular, if no block were transmitted (noise alone), the quality indicator for the mobile station in question degrades, so that it undergoes a decrease in useful bit rate, although it perhaps benefits from excellent radio conditions.
The link adaptation techniques are not the only ones that may be disturbed through the false detection of data blocks. For example, such is the case also for automatic block repetition (ARQ) techniques with incremental redundancy (IR). In these techniques, the data of a block badly decoded by the receiver are preserved by the latter and a repeat thereof is requested. The data preserved then enrich those obtained following the repeat so as to increase the probability of detection. It is important to prevent a block from being detected in error in the presence of noise alone since this may introduce confusion into the controlling of the sending of blocks.
These drawbacks may be attenuated by increasing the size of the CRC of the header. However, this is to the detriment of the transmission bit rate, and is not possible in a system whose formats are already standardized.
An object of the present invention is to minimise the risks of false detection of such blocks having several codes for error detection.